Periodic antenna adapted for handling high power

ABSTRACT

A periodic antenna includes a plurality of quarter wavelength resonant, radiating elements spaced along a feed line positioned above a ground plane. Adjacent ones of the resonant elements are separated from each other by a distance substantially less than a quarter wavelength of the operating frequency of the elements and are positioned to extend from the feed line in a direction away from the ground plane. Substantially 180*phase shift is provided between adjacent ones of the elements by including quarter wavelength, shunt distributed parameter slow-wave delay lines between adjacent ones of the elements. The delay lines are located inside of an electrically conducting sleeve that comprises the feed line.

Brunner [4 1 Apr. 30, .1974

[ PERIODIC ANTENNA ADAPTED FOR HANDLING. HIGH POWER John E. Brunner,Hamilton, Ohio [73] Assignee: Cincinnati Electronics Corporation,

Evendale, Ohio [22] Filed: Nov. 29, 1972 [21] Appl. No.: 310,543

[75] Inventor:

[52] US. Cl 343/792.5, 343/846, 343/853 [5l] Int. Cl. .Q H0lq 11/10 [58]Field of Search 343/792.5, 816, 846, 853

[56] 0 References Cited UNITED STATES PATENTS 3,389,396 6/1968 Minervaet al 343/792.5

Primary Examiner-Eli Lieberman Attorney, Agent, or Firm-Lowe, King andPrice [57] ABSTRACT A periodic antenna includes a plurality of quarterwavelength resonant, radiating elements spaced along a feed linepositioned above a ground plane. Adjacent ones of the resonant elementsare separated from each other by a distance substantially less than aquarter wavelength of the operating frequency of the elements and arepositioned to extend from the feed line in a direction away from theground plane. Substantially l80phase shift is provided between adjacentones of the elements by including quarter wavelength, shunt distributedparameter slow-wave delay lines between adjacent ones of the elements.The delay lines are located inside of an electrically conducting sleevethat comprises the feed line.

10 Claims, 2 Drawing Figures MTENTEmmo m4 sum 1 or 2 PERIODIC ANTENNAADAPTED FOR HANDLING HIGH POWER FIELD OF INVENTION The present inventionrelates generally to periodic antennas and more particularly to aperiodic antenna including improved means for providing 180 phase shiftbetween adjacent resonant radiating elements of the antenna.

BACKGROUND OF THE INVENTION One type of periodic antenna includes a feedline and a plurality of distinct resonant radiating elements that areconnected to and spaced along the feed line. To provide a directive,relatively high ga'in'pattern, rather than a substantiallyomnidirectional low gain pattern, it is necessary to provide 180 phaseshift between adjacent radiating elements along the feed line. A 180phase shift between adjacent radiating elements is required for adirectional pattern regardless of whether the periodic antenna is narrowband, i.e., uniformly periodic, or wide band, i.e., log-periodic. In auniform periodic antenna, all of the radiating elements have the samelength and are equally spaced from each other, while in a log-periodicantenna adjacent elements along the feed line have differing lengths andspacings. In a log-periodic antenna, the ratio of the length of adjacentelements equals the ratio of the spacing of adjacent elements from anapex of the antenna, which ratio is referred to as the structuralscaling parameter of the antenna and denoted by 1'.

Many systems have been developed in the prior art for providing the 180phase shift between adjacent resonant radiating elements. Perhapsthemost well known involves periodic antennas including a plurality ofhalf wavelength dipole radiating elements which are-interconnected by atwo-conductor common feed line, transposed between connections. Theelements and feed are interconnected in such a manner that adjacentquarter wavelength elements on the same side of the line are connectedto opposite leads of the feed. However, for many applications the bulkrequired by a half wavelength dipole is not practical and the half wavedipole configuration is replaced by monopole, quarter wavelengthresonant radiating elements and a ground plane conductor.

Several different structures for providing 180 phase shift betweenadjacent monopole radiating elements have been proposed; namelyone-to-one phase reversing transformers, coaxial capacitors, groundedparasitic elements located between adjacent active monopoles, lumpedparameter resonant circuits between adjacent monopoles and quarterwavelength, resonant lines between adjacent radiating monopoles. Eachsuggested prior art device of these classifications has had problems,particularly in applications requiring high power (approximately 1kilowatt), low frequency over a wide bandwidth (such as 4-30 MHz), andtransportability. Transformers are cumbersome, expensive and notparticularly well suited for high power applications. Parasitic elementsare relatively narrow bandwidth structures and, therefore, function foronly a small range of array scaling parameters.

While coaxial capacitors combine simplicity and high power capability,they are not practical if relatively small spacing between adjacentelements is required as is usually the case for an easily transportedantenna capable of operating over a wide bandwidth and with a broadbeamwidth. For these requirements, it is neces' sary to select a spacingbetween adjacent elements that is a relatively small fraction (0.06 orless) of a wavelength. For spacings between adjacent elements equal toor less than 0.06 wavelength, the tolerance of coaxial capacitors isusually excessively critical.

It has been proposed to provide spacings between adjacent elements of0.06 wavelengths or less by including series resonant elements orquarter wave resonant lines. However, the proposed structures have notbeen adequate to meet the high power and transportability requirements.A series tuned circuit includes lumped parameter inductors having veryhigh Q. These inductors are required to conduct significant amounts ofcurrent when operated near resonance and, therefore, have a significant1 R power loss which is heat that must be dissipated. Heat dissipationof the inductors has generally been attained by packaging them inrelatively large housings which detract from the ability of the antennato be moved from one place to another.

To avoid the heat dissipation problem of the lumped parameter inductor,it has been suggested to use distributed parameter quarter wavelengthtransmission lines which extend from and are connected to either side ofthe feed in a plane generally parallel to the ground plane. The amountof space required for such an array is relatively great because of thelong elements extending outwardly from the feed. In certain regions,sufficient area cannot be cleared to enable the antenna to functionproperly. In the specific frequency range mentioned, approximately feetto the sides of the feed element are required for the longest quarterwavelength lines.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the presentinvention, the phase shift between adjacent resonant radiating elementsof a periodic antenna is provided by forming the feed transmission-linefrom a hollow electrically conductive sleeve in which distributedparameter, quarter wavelength slow-wave structures are provided. Each ofthe slow-wave structures is preferably a helical transmission linehaving distributed capacity between adjacent turns and an inner wall ofthe hollow conductive sleeve. One end of the transmission line isconnected, via a small trimer capacitor, to the conducting sleeve, whilethe other end is connected to the ground plane against which the feedline is driven, between adjacent pairs of radiating elements for whichthe slow-wave delay line is tuned. The slow-wave delay line does notrequire a relatively massive container for dissipating heat generatedtherein since the heat is distributed over the entire length of theline, rather than being concentrated in a small volume, as is the casewith an inductor of a lumped parameter shunt resonant circuit. Hence,the slow-waver, distributed parameter delay line is particularly suitedfor relatively high power applications. Because the slow-wavedistributed parameter quarter wavelength line is located within theinterior of the electrically conductive sleeve, the unit is relativelytransportable and does not require a significant area on the terrain inwhich it is located. For the high power applications of the presentinvention, it is not feasible to utilize a lumped parameter inductanceinside the conducting sleeve because eddy currents of the sleeve reducethe Q of the coil and increase losses, and may cause excess heatdissipation.

To provide vemier tuning for the helical delay line, a distributedparameter capacitor is provided in series with the helix. Thedistributed parameter capacitor comprises a solid dielectric having oneelectrode formed by the conductive sleeve and a second electrodeconnected to one end of the helical line.

It is, accordingly, an object of the present invention to provide a newand improved periodic antenna which is capable of operating atrelatively high power, at a relatively low frequency, is relativelytransportable, can easily be erected without requiring excessive amountsof ground area, and does not exhibit excessive heating.

An additional object of the invention is to provide a periodic antennawherein quarter wavelength elements are located inside of a feed linefor a number of resonant radiating elements.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of oneembodiment of the present invention; and

FIG. 2 is a cross-sectional view of a portion of the feed lineillustrated in FIG. I, particularly illustrating one quarter wavelengthslow-wave structure.

DETAILED DESCRIPTION OF THE DRAWING Reference is now made to FIG. 1 ofthe drawing wherein there is illustrated an antenna array comprising aplurality of monopole, quarter wavelength radiating elements 11-12 whichhave differing lengths and spacings between adjacent elements inaccordance with the well known log-periodic criteria, to satisfy theantenna scaling factor R =distance from apex point 23 to one of theelements (n l as measured along boresight axis 24,

R distance from apex point 23 to an element (n) adjacent the element (nl) as measured along boresight axis 24, L l length of element (n l) asit extends from boresight axis 24, and L length of element (n) as itextends from boresight axis 24. Elements 11-21 are fed from a commonfeed line 25, having a longitudinal axis coincident with boresight axis24. The spacing between adjacent ones of elements ll-21 is considerablyless than a quarter wavelength of the resonant frequency of each elementof the pair, and is typically between 0.02 and 0.06 of a wavelength toprovide a relatively short length for feed element 25. Elements 11-21are positioned to extend from feed line 25 in a direction at rightangles to the plane of ground plane 26, above which the feed line ispositioned.

Feed line 25 is situated slightly above ground plane 26 so that there isa radiation image of each of the radiating element 11-21 in the groundplane, whereby the array of monopole elements appears to include anarray of half wavelength dipole elements.

Connected to the end of common feed line 25 adjacent the shortestmonopole radiator 11 is an active, generally wide band, such as 4-30MHz, source 27. Substantial energy from source 27 is transmitted alongfeed line 25 until it is coupled to the radiator with which it is mostnearly resonant. The monopole elements having a length less than thewavelength of the energy from source 27 present a relatively highimpedance to the source energy and do not, therefore, absorb substantialenergy from the source. The monopole elements which have a lengthenabling them to be resonant with the frequency of source 27 radiateconsiderable energy from the source, and considerably attenuate thesource energy to cause a reversal in slope of the propagation constantalong the feed line 25. Energy radiated from the element most nearlyresonant with source 27 normally has a tendency to be radiated in bothdirections relative to boresight axis 24, i.e., there is a tendency togenerate lobes that are respectively directed toward the end of feed 25to which source 27 is connected and toward the end of the feed where thelongest radiating monopole 21 is located. To suppress the lobe that hasa tendency to radiate toward monopole 21 and provide a substantialbackfire radiation pattern, a 180 phase shift is provided betweenadjacent ones of radiating elements 11-21.

The description and method of operation of the structure illustrated inFIG. 1 is well known to those skilled in the art. The present inventionis concerned with a new and improved apparatus for achieving the 180phase shift between adjacent ones of radiating elements 11-21. I

To this end, common feed line 25 is formed of a hollow electricallyconductive, non-magnetic sleeve, which is preferably fabricated ofaluminum. Within sleeve 25 are located a plurality of quarterwavelength, slow-wave shunt distributed delay lines 31-40, one of whichis provided for each pair of adjacent radiating elements 11-21. Hence,slow-wave delay line 31 is provided for radiating elements 11 and 12,slow-wave delay line 32 is provided for radiating elements 12 and 13,etc. and slow-wave delay line 40 is provided for radiating elements 21and 20. Each of delay lines 31-40 is designed so that the resonantfrequency thereof, the frequency at which a phase shift is provided, isequal to the resonant frequency of the longest radiator of the radiatorof its associated pair divided by VT For example, slow-wave delay line40 is designed to provide a 90 phase shift for the resonant frequency ofradiator 21 divided by [T One end of each of quarter wavelength delayline 31-40 is connected to any point on common feed line 25, while theother end of each quarter wavelength delay line is connected to groundplane 26 at a point that is at the geometric means between the twoadjacent radiating elements of each radiating pair, as referenced toapex point 23. Hence, for radiating pairs having lengths L and L spacedfrom apex point 23 by the distances R, I and R, the quarter wavelength,slow-wave structure has one terminal connected to ground plane 26 at apoint spaced from a'pex point 23 along boresight axis 24 by a distanceequal to V l R,,l

(R, If radiating elements 11-21 are not logarithmically periodic but areuniformly periodic, whereby the spacings between adjacent elements andthe lengths of all elements are the same, the connection point of eachquarter wavelength slow-wave structure to the ground plane isequidistant from adjacent radiating elements. While the one end of thequarter wavelength, delay line structures 31-40 can be connected at anypoint to feed line 25, it is usually convenient to connect the slowwavestructures to the feed line in proximity to the longest radiatingelement of a particular radiating element pair.

One particular configuration for each of the slowwave, quarterwavelength, shunt distributed parameter delay lines located within feedline 25 is illustrated in P16. 2 and broadly comprises a helical coil 41and a vemier capacitor 42. Each of the distributed parameter quarterwavelength delay lines 31-40 is essentially the same, except for thelength of the helical conductor and the value of the vernier capacitor,which enables the capacitance of the helical conductor to change byapproximately fl percent. I a

As illustrated in FIG. 2, helical conductor 41 is mounted inside ofaluminum sleeve 43, which comprises feed line 25, and to which aresoldered radiating element s 1l-21. Helical conductor 41 is supported bydielectric tube 44 which is coaxial with sleeve 43. Tube 44 is supportedand maintained in situ by spaced dielectric rings 45 and 46 which arepositioned in proximity to the opposite ends of helical conductor 41.

The end of helical conductor 41 proximate ring 46 is capacitivelycoupled to metal sleeve 43 by capacitor 42 which comprises annular,solid dielectric cylinder 47, having inner and outer peripheriesrespectively in contact with tube 44 and sleeve 43. Electrodes ofcapacitor 42 are respectively formed by sleeve 43 and metal tube 48which is nested within tube 44 so that the outer surface of the formerabuts against the inner surface of the latter. Tube 48 and dielectriccylinder 42 have approximately equal lengths to provide the greatestvariation of the vemier capacitor, the value of which is controlled bythe longitudinal position of tube 48 relative to cylinder 42. Theelectrode of capacitor 42 formed by metallic tube 48 is connected to oneend of helical conductor 41 by metallic plate 49 that is fixedly bondedto the end of tube 48 closest to ring 46.

The other end of helical conductor 41 is connected to ground plane 26which comprises a metallic threshold strip 51 having a length equal tothe length of tube 43 and which is adapted to be placed on the terrainon which the antenna is erected. To this end, helical conductor 41includes, at its end remote from plate 49, a straight segment 52 whichextends through dielectric grommet 53 that is provided in an aperture oftube 43. To support the antenna, a dielectric leg 54 surrounds grommet53 and straight segment 52 of helical coil 41 and is fixedly connectedbetween the exterior of sleeve 43 and metallic threshold 51. Leg 54spaces sleeve 43 from threshold 51 by a distance which affects thecharacteristic impedance of the feed line and, therefore, the entirearray. For a typical situation where it is desired for the antenna toexhibit an input resistance of 50 ohms, the spacing of the bottom ofsleeve 43 from threshold 51 is on the order of two and one-half inches.

It is to be understood that in certain instances the distributedcapacitance between the helical coil 41 and hollow conductive tube 43and the coil length are such that the distributed parameter slow-wavestructure is resonant to exactly the correct frequency, whereby the needfor a vernier capacitor is obviated. Precise control of the phase shiftintroduced by the slow-wave structure can be achieved by accuratelycontrolling the length, pitch and diameter of helical conductor 41.Precise control of these parameters can be achieved by providingdielectric tube 44 with an accurately machined helical groove in whichthe helical coil is wound.

One particular configuration of the antenna of the present invention wasdesigned to cover a bandwidth from 4 to 30 MHz and included thedistributed parameter helical delay line configuration illustrated inFIG. 2. To provide a relatively short structure, the value of 'r wasselected to be 0.900, and the spacing, generally denoted by 8, betweenadjacent radiators 11-21 was selected to be 8 0.045 A, where A thewavelength of the resonant frequency of the longest element of a pair ofthe adjacent elements. The length of sleeve 43 in this configuration was98.5 feet, approximately a 50 percent reduction relative to standardlog-periodic antennas covering the same frequency band. Aluminum sleeve43 had a 1.75 inch diameter and a wall thickness of 0.049 inches, whiledielectric tube 44 was fabricated from fiberglas having a 1.00 inchouter diameter and a 0.050 inch wall thickness. Helical center conductorwas fabricated from number eight copper wire and wound to have 4.5 turnsper inch, a spacing that was found to provide the highest Q per unitlength for the relatively small 1.129 inch diameter between the centersof the wire coil on opposite sides of tube 44. The length of helicalcoil 41 was between 5 inches and 4 feet, depending upon which of thepair of resonant monopoles with which it was associated. The describedantenna exhibited a gain of between 8 and 9 db relative to a tunedquarter wavelength monopole and a voltage standing wave ratio of lessthan two to one over the entire band. Testing of the antenna at powerlevels up to one kilowatt indicated no evidence of arcing or excessiveheating of the distributed parameter delay lines.

While a helical distributed parameter delay line is most advantageousfor relatively low frequency bands, the principles of the invention areapplicable to distributed parameter delay lines comprising astraight-line conductor in a high dielectric medium. ln such aninstance, the helical center conductor 41 is replaced by a straight-lineconductor that is coaxial with tube 44 and extends longitudinallythereof. The straight-line conductor is surrounded by a solid dielectrichaving a relatively high dielectric constant, such as certain ceramics.

The principles of the invention are also applicable to any distributedparameter slow-wave delay line elements which introduce an integralmultiple of a quarter wavelength delay between common feed element 25and ground plane conductor 26. The quarter wavelength structure,however, is preferable to higher multiples of a quarter wavelengthbecause of its shorter length and lower losses.

While there has been described and illustrated one specific embodimentof the invention, it will be clear that variations in the details of theembodiment specifically illustrated and described may be made withoutdeparting from the true spirit and scope of the invention as defined inthe appended claims.

I claim:

1. A periodic'antenna particularly adapted for han-' dling high powercomprising at least three resonant radiating elements, a common feedline for said elements, said feed line comprising a hollow electricallyconducting, metallic sleeve extending along substantially the entirelength of the line, adjacent ones of said elements being spaced alongsaid feed line by a distance substantially less than a quarterwavelength of the operating frequency of the elements, means forproviding substantially 180 phase shift between adjacent ones of theelements, each of said phase shift means including a shunt distributedparameter slow-wave delay line connected between adjacent ones of saidelements and located inside of the sleeve, said line including aconductor having distributed inductance for heat distribution over itsentire length and distributed capacitance as a dielectric between theconductor and sleeve, each of said shunt lines being spacedlongitudinally in the sleeve from the adjacent line and tuned so it hasa length substantially equal to an integral multiple of a quarterwavelength of the resonant frequency of a radiating element adjacentthereto.

2. The periodic antenna of claim 1 wherein each of the shunt linesincludes a helical coil coaxial with the sleeve and having a lengthsubstantially equal to a quarter wavelength.

3. The periodic antenna of claim 2 wherein the shunt line furtherincludes a tuning capacitor for precisely determining the resonantfrequency of the shunt line.

4. The periodic antenna of claim 3 wherein the capacitor includes firstand second electrodes respectively formed by the hollow, electricallyconducting sleeve and an electrically conducting tube coaxial with theconducting sleeve, and an annular, solid dielectric cylinder positionedbetween said electrodes.

5. A periodic antenna particularly adapted for handling high powercomprising a ground plane, a feed line positioned above the groundplane, at least three monopole resonant radiating elements spaced alongthe ground plane conductor and positioned to extend from the feed linein a direction away from the ground plane, said feed line and elementsbeing positioned above the ground plane so that there is a radiationimage of each radiating element in the ground plane, said feed linecomprising a hollow, electrically conducting, metallic sleeve extendingalong substantially the entire length of the line, adjacent ones of saidelements being spaced along said feed line by a'distance substantiallyless than a quarter wavelength of the operating frequency of theelements, means for providing substantially phase shift between adjacentones of the elements, each of said phase shift means including a shuntdistributed parameter slow-wave delay line connected between adjacentones of said elements and located inside of the sleeve, said lineincluding a conductor having distributed inductance for heatdistribution over its entire length and distributed capacitance as adielectric between the conductor and sleeve, each of said shunt linesbeing spaced longitudinally in the sleeve from the adjacent line andtuned so it has a length substantially equal to an integral multiple ofa quarter wavelength of the resonant frequency of a radiating elementadjacent thereto.

6. The periodic antenna of claim 5 wherein the ground plane includes ametallic threshold strip, and the shunt line includes first and secondterminals of the conductor respectively connected to the feed line andthe ground plane conductor, said second terminal being connected by adc. connection to the threshold strip between the resonant elements forwhich the delay line is tuned.

7. The periodic antenna of claim 6 wherein each of the shunt linesincludes a helical coil coaxial with the sleeve and having a lengthsubstantially equal to a quarter wavelength.

8. The periodic antenna of claim 7 wherein the shunt line furtherincludes a tuning capacitor for precisely determining the length of theshunt line.

9. The periodic antenna of claim 8 wherein the capacitor includes firstand second electrodes respectively formed by the hollow, electricallyconducting sleeve and an electrically conducting tube coaxial with theconducting sleeve, and an annular, solid dielectric cylinder positionedbetween said electrodes.

10. The periodic antenna of claim 5 wherein the lengths of said elementsdiffer from each other and the spacing relative to a common apex pointbetween adjacent ones of said elements differs in accordance with thelog periodic criteria, said ground plane includes a metallic thresholdstrip, said shunt line including first and second terminals of theconductor respectively connected to the feed line and the ground planeconductor, said second terminal being connected by a dc. connection tothe threshold strip between the resonant elements for which the delayline is tuned at a point spaced from the apex by substantially thegeometric mean of the distance from the apex of the adjacent elements towhich the shunt line is tuned.

1. A periodic antenna particularly adapted for handling high powercomprising at least three resonant radiating elements, a common feedline for said elements, said feed line comprising a hollow electricallyconducting, metallic sleeve extending along substantially the entirelength of the line, adjacent ones of said elements being spaced alongsaid feed line by a distance substantially less than a quarterwavelength of the operating frequency of the elements, means forproviding substantially 180* phase shift between adjacent ones of theelements, each of said phase shift means including a shunt distributedparameter slow-wave delay line connected between adjacent ones of saidelements and located inside of the sleeve, said line including aconductor having distributed inductance for heat distribution over itsentire length and distributed capacitance as a dielectric between theconductor and sleeve, each of said shunt lines being spacedlongitudinally in the sleeve from the adjacent line and tuned so it hasa length substantially equal to an integral multiple of a quarterwavelength of the resonant frequency of a radiating element adjacentthereto.
 2. The periodic antenna of claim 1 wherein each of the shuntlines includes a helical coil coaxial with the sleeve and having alength substantially equal to a quarter wavelength.
 3. The periodicantenna of claim 2 wherein the shunt line further includes a tuningcapacitor for precisely determining the resonant frequency of the shuntline.
 4. The periodic antenna of claim 3 wherein the capacitor includesfirst and second electrodes respectively formed by the hollow,electrically conducting sleeve and an electrically conducting tubecoaxial with the conducting sleeve, and an annular, solid dielectriccylinder positioned between said electrodes.
 5. A periodic antennaparticularly adapted for handling high power comprising a ground plane,a feed line positioned above the ground plane, at least three monopoleresonant radiating elements spaced along the ground plane conductor andpositioned to extend from the feed line in a direction away from theground plane, said feed line and elements being positioneD above theground plane so that there is a radiation image of each radiatingelement in the ground plane, said feed line comprising a hollow,electrically conducting, metallic sleeve extending along substantiallythe entire length of the line, adjacent ones of said elements beingspaced along said feed line by a distance substantially less than aquarter wavelength of the operating frequency of the elements, means forproviding substantially 180* phase shift between adjacent ones of theelements, each of said phase shift means including a shunt distributedparameter slow-wave delay line connected between adjacent ones of saidelements and located inside of the sleeve, said line including aconductor having distributed inductance for heat distribution over itsentire length and distributed capacitance as a dielectric between theconductor and sleeve, each of said shunt lines being spacedlongitudinally in the sleeve from the adjacent line and tuned so it hasa length substantially equal to an integral multiple of a quarterwavelength of the resonant frequency of a radiating element adjacentthereto.
 6. The periodic antenna of claim 5 wherein the ground planeincludes a metallic threshold strip, and the shunt line includes firstand second terminals of the conductor respectively connected to the feedline and the ground plane conductor, said second terminal beingconnected by a d.c. connection to the threshold strip between theresonant elements for which the delay line is tuned.
 7. The periodicantenna of claim 6 wherein each of the shunt lines includes a helicalcoil coaxial with the sleeve and having a length substantially equal toa quarter wavelength.
 8. The periodic antenna of claim 7 wherein theshunt line further includes a tuning capacitor for precisely determiningthe length of the shunt line.
 9. The periodic antenna of claim 8 whereinthe capacitor includes first and second electrodes respectively formedby the hollow, electrically conducting sleeve and an electricallyconducting tube coaxial with the conducting sleeve, and an annular,solid dielectric cylinder positioned between said electrodes.
 10. Theperiodic antenna of claim 5 wherein the lengths of said elements differfrom each other and the spacing relative to a common apex point betweenadjacent ones of said elements differs in accordance with the logperiodic criteria, said ground plane includes a metallic thresholdstrip, said shunt line including first and second terminals of theconductor respectively connected to the feed line and the ground planeconductor, said second terminal being connected by a d.c. connection tothe threshold strip between the resonant elements for which the delayline is tuned at a point spaced from the apex by substantially thegeometric mean of the distance from the apex of the adjacent elements towhich the shunt line is tuned.